Picture and sound transmission



y 1938- c. o. BROWNE ET AL." 2,124,394

PICTURE AND SOUND TRANSMISSION Filed Aug. 10, 1934 A rromvsy.

Patented July 19,1938 i 2,124,394

UNITED sTATEs PATENT OFFICE PICTURE AND: soUNb TRANSMISSION Cecil OswaldBrowne, West Acton, London, and

John Hardwick, West Drayton, England, assignors to Electric and MusicalIndustries Limited, Middlesex, England, a company of Great BritainApplication August 10, 1934, Serial No. 739,230 In Great Britain August14, 1933 .4. Claims. (01. its-5.8)

"Ifhe present invention relates to televisionand will allow to pthrough, t putting the inthe transmission of pictures with soundsapproeideht light equal to u ty. We have priate thereto. 1

It is the aim, in most known television sys- I D=log 7 terms, to produceat the receiver an image of v which the brightnesses of all points areexactly From these two equations it may be easily shown proportional tothe intensities of the light emathat the intensity of the transmittedlight is hating from corresponding points of the object equal to theintensity of the exposure raised to at the transmitter. the power .'yand after reversal by printing we 10 It has long been recognized in thecinema art, have, for the print, 10

however, that if the brightnes'ses of all points of fi an image viewedon a projection screen are proportional to the brightnesses on theobject from N w if I be plotted as ordinate against E which the imagesare derived, the screen image a b i nd v be given the value unity, a

ppears flat n unin ere ing to th y This straight line is obtainedindicating that for all 15 effect is beli v t be due to h feet thatethepoints of the pictures on the positive print the images e projected n bac a White and h intensity of the transmitted light is proportionaladditional efiect of detail Which would be giv n to the intensity of theexposing light, that is to to the lighter parts of the pi r y theIlatsay, is proportional to the brightness of corre- 2 ural colours isabsent. It is therefore common ndin int n the object, 0

practice in the i m ry to ev p plc- But if 'y is given the value 2, aparabolic curve ture films in such manner'tha deta in hel ehtsymmetricalabout the I axis is obtained. This er or'less Op q p n of the positivefi curve indicates that for small values of I or E pictures broughtamplified as it Were, a small change inE produces a small change inrelatively to the detail in the darker or more I but for large values ofI andE a small change 25 opaque portions of the pictures A filmdeveloped in E produces a comparatively large change in n his w y is ito b vel p o a gamm I. A positive film developed so that 'y'=2 thusintensity contrast of Value greater than shows little detail in darkportions of the picun ty a d, When p je up n a Screen, the tures andmuch detail in the brighter portions.

images derived from such a film are more pleas- On the other hand if vbe given the v l 30 inc and appear m re full f d a t a images paraboliccurve symmetrical about the E-aXis is p j d from a film of intensity traequal obtained and a positive film developed to this t y, that is to y,a film in Which the value of v shows greater detail in dark portionslight transmitted through all points is exactly of t picture th inlighter portions,

pr porti n to he ri h nesses of h corre- Thus by suitably choosing theWay in which a 35 sponding object points. I film is developedconsiderable control is obtained It is found in fact that a fildeVeloped to over the relative detail in the dark and lightporanintensity contrast of fromabout 1.8 to 2.0 is tions of th pictures onthe film, most sat s y in this respect?- Now it has been found that if,in a television The mannerin which t e d v p Of a system, an object ,isscanned and the picture 40 film affects the finally reproduced imagesmay signals amplified in a thermionic amplifier, a be appreciated moreclearly from the following parasitic background may be produced in theconsiderations: amplifier by the well-known Johnson noise,

The optical density (D) of any point on a which is Caused mainly by thep lg resistnegative after development is proportional to ances i P earlyof the amphfier: m 45 the logarithm to the base In of the intensity ofalso b shght Vanatlons m the anode Currfnt 1n the light (E) to whichthat point was exposed. fi ira of the amphfier a Shot;

- v e co is may cause a parasl 10 ac groun Thus We have. themathematlca'l expresslon to appear in the reconstituted pictures at theD='y10g10E, receiver. A further and even more objectionable 50 form ofparasitic background at the receiver is Where represents a Constant- TheOptlcal that due to stray radio interference which ar- S y of y point011 a det'eloped negative may rives at the detector of the receiver withthe a so b measured y the ratio of the incident television signals.Parasitic background due to light to the transmitted light (I) which thepoint any of the causes mentioned is only Objectionable 55 According tothe present invention a method of transmitting an image of an object'to' a distance comprises the steps of generating electrical picturesignals representative of the light and shade of the object to betransmitted, reducing the effective intensity contrast of said picturesignals before transmission and increasing the effective intensitycontrast of said signals at the receiver.

The present invention further provides a methbe obtained by increasingthe effective intensity contrast at the receiver. When sounds aretransmitted as an accompaniment to the pictures, the eifective intensitycontrast of the sound signals is preferably maintained at substantiallyunity throughout the system.

Other features of the invention will be apparent from the followingdescription andthe appended claims.

The invention will be described by Way of example with reference to theaccompanying drawing in which Figs. 1 and 2 are schematic diagramsillustrating a transmitter and a receiver respectively according to theinvention and Figs. 3 and 4 are explanatory curves.

Referring to Fig. 1, there is shown diagrammatically an embodiment ofthe invention as applied to the transmission of a talking motion picturefilm. V V r The pictures and sounds: may be recorded upon the film Ifrom which transmission is to be effected in any known or suitablemanner. It will be assumed that the sounds are recorded by the varyingdensity, constant width method.

At any convenient time after the recording processes have been completedthe sound and picture records are developed to a gamma or intensitycontrast of unity (in contradistinction to the known methods where thefilm is developed to an intensity contrast of approximately 1-8) and apositive print prepared. The light transmitted by the positive, afterdevelopment, is thus proportional at all points to the intensity of theexposing light, that is to say, to the intensity of the light emanatingfrom the object.

With the foregoing explanation in mind, it will be readily appreciatedthat, in comparison with the films used hitherto for purposes ofcinematography and television transmission, the detail in the darkportions of the present positive film is emphasized relatively to thedetail in the lighter portions of the film.

The picture record I is then scanned by moving the film uniformlydownwards past a picture scanning gate 2, in front of a light source 3.An image of the gate or slit 2 is reflected by a mirror of the mirrordrum 4 and focused by a lens system represented by 5 upon anaperture 6behind which is a photo-cell I. The film I is also moved a at uniformspeed past a sound gate 8 whilst a beam of light from a source 9 ofconstant intensity is passedthrough the sound track on the film into asecond photo-cell Ill. The photo-cells I and ID are of normal type andhave a substantially linear response, that is to say their effectiveintensity contrast is substantially unity.

The picture signals from the-cell I are amplified in a thermionic valveI I and further in an amplifier representedby the rectangle I2 and arethen fed to a transmitter I3. The valve II may be operated on thestraight part of its anode current (IQ-grid voltage (Es) characteristic,that is to say it may be biased to a point such as I4 in Fig. 3 whichshows one such characteristic.

-It should be noted that the picture signals will cause excursions fromthe point I4 only in a downward direction since picture signalcomponents down to and including those of effectively zero frequency,are fed to the valve II and since the point I4 represents pictureblackfBecause the electrical variations used to modulate the transmitterI3 are proportional tothe light and shade ofthe pictures upon the filmI, a given change of picture light intensity, whether in the darker orlighter parts ofrthe picture, will be represented by the same change ofpicture signal voltage, Thus the effective intensity contrast of thepicture signals transmitted may be said to be the same as that of thefilm, namely unity.

The sound signals from the cell III are amplified substantially withoutdistortion in an amplifier I5 and are fed to'a transmitter I6 which mayoperate upon a difierent, wave length from the picture transmitter I3.

- a receiver 2 I, the sound signals are amplified substantially withoutdistortion in an amplifier 22 and fed to a reproducer 23. V It hasalready been pointed'out that the parasitic background is noticeableonly if the amplitude of-the picture signals remains at a valuecomparable with that of the background; in known systems this occursbecause the film is developed to an intensity contrast of 1'8 thusreducing the amplitude of signals corresponding to darker portions ofthe picture; in the present case, however, the amplitude of signalscorresponding to any given intensity change in the darker portions ismaintained at the same value as that of the lighter portions owing tothe development'being'taken to a gamma of unity. The parasiticbackground may thus be made unnoticeable in both light and dark parts ofthe picture. The point may perhaps be made still clearer by thefollowing example. There will be considered three points on an objecthavingintensities 11, I2 and I3 respectively and the values of thesethree intensities will be assumed to be 100, 10 and 1 respectively. Ifthe film is such that 'y=1 and assuming a linear photo-cell is used, thecorresponding picture currents will be proportional to 100, 10 and 1.respectively. But if 7:2, say, the currents 1 and; p01 respectively.whenthe highest intensity signal is represented by 100 as before, andit will be .clear that any parasitic disturbance will have, a far more.serious effect when the low intensity signal is represented by anamplitude 0-01 than when it is represented by an amplitude equal tounity as will bethe case when the signals are transmitted with =1. i v.1 i 1 It should be noted,howe ver, that when .the picture signals aretransmitted with an effectiveintensity contrast of unityithe wave formof the amplified picture signals at the-receiver is a substantiallyfaithful representation of a picture having an intensity contrast ofunity. If, therefore, these signals are reconverted, without distortion,into pictures,the latter will be of the fiat, uninteresting type.mentioned above. The device 19 is therefore arranged to increasetheeffective intensity contrast of the received signals.

The cathode ray tube shown diagrammatically at l9 may compriseanindirectly heated cathode 24, amodulating or grid electrode 25 placedclose to the cathode, one. or more focusing electrodes 26,;2'Lan. anode28 in additionto the fluorescent screen 20.- Means represented at 29 arealso provided for deflecting the ray over the fluorescent screeninsynchronism with the scanning operation at the transmitter, suitablesynchronizing signals being generated and sent from the transmitter soas to holdthe scanning processes in synchronism in known manner,

The picture signals are applied as already described between the grid 25and cathode 24 so as to modulate the intensity of the ray, and thus theintensity of fluorescence, in accordance with the wave form of thepicture signals.

Now it is found that if the screen current (I5) of the tube be plottedagainst the grid volts (Vg) a curve is obtained which can be representedby the equation IS Vg where A represents a constant. Thetube may thus besaid to have an equivalent intensity contrast represented by A in theabove equation; For small values of Is or V a small changein Vg producesa small change in Is but for large values of Is or Vg a small change inVg produces a relatively large'cha nge in Is. Thus in normal tubes A isalwaysgreater than unity and usually has a value of about 2. By suitabledesign and with suitable working voltages on its electrodes the tube canbe given a characteristic in which A has the value required to raise theeffective intensity contrast from unity to the desired higher value.

Thus, in so far as the pictures are concerned, the system as a whole canbe made effective in rendering the parasitic background effectsrelatively imperceptible and in producing pictures of intensity contrastin the neighbourhood of 1-8.

The received sound signals are reconverted into sounds in any known orsuitable manner and substantially without distortion so that the overallintensity contrast of the system, in so far as the sounds are concerned,is 1-0 and the sounds remain substantially undistorted throughout thetransmission.

If it is required to use a cathode ray tube 19 or other reconstitutingdevice having too low an effective intensity contrast for satisfactorypicture production with signals having an effective intensity contrastof unity, the valve l8 can be arranged to increase the effectiveintensity contrast of the signals before they are fed to the device l9.This can be done by biasing the valve l8 to a point such as 30 in Fig.4, (it being assumed that picture signaLcomponents down'to-andincludingthe direct component are arrangedv to be present). Sincevoltage excursions of the grid :of thevalve l8 mustbearranged to bedownward along the characteristic it .will be clearthatsignals of smallamplitude are amplified less than signals of greater amplitude andtheeffective intensity contrast of the valve l 8 worked in thisway willtherefore be greater. than unity. The valve utilized to increase theeffective intensity con,- trast is preferably that immediately precedingthe reconstituting device in order that the maximum benefits of .alowintensity, contrast in reducing parasitic background may be obtained.

Similarly if, the device I9 has an unduly high effective intensitycontrast, the. latter may-be reduced by operating the valve H5 at apoint such as point 3| in Fig. 3.

If instead of a cathode ray tube at the receiver there is used a devicesuch as a neon tube having an effective intensity contrast approximatelyequal to unity, the effective intensitycontrast of the signals may beincreased to the desired value as above described before the signals area'oplie to the device.

Ithas been foundthat other devices, such as Kerr and Faraday cells,operate in a manner similar to a cathode ray tube in that during theconversion of electric picture signals into light values, they tend toamplify signals corresponding, to light portions relatively tosignalscorresponding to darker'portions. Such devices may thus be said to havean equivalent intensity contrast greater than unity and maybe used inthe above described system in place of the cathode ray tube.

Clearly by a suitable choice of theshape, of the lower or upper curvedportions of the characteristic curve of the valve by which ;a change ineffective intensity contrast is eifected and by suitable design andadjustment of the valve, the

' amplifier maybe arranged in any particular case to give the desiredoverall effective intensity'contrast.

If desired the picture signals may be transmitted with aneffectiveintensity contrast less than unity. For this purpose if theobject to be transmitted is a film, the film may be developed to anintensity contract less than unity or a film of intensity contrastgreater than unity may be employed and the effective intensitycontrastreduced at the transmitter, for example with the aid of thevalve II in Fig. 1. Thus the valve ll may be biased to a point such as3| in Fig. 3, the excursions of grid voltage being as before downwards.By the use of a suitable correcting device, a film of normal intensitycontrast, such as about 1-8, may be used for transmission and theeffective intensity contrast may be reduced in the correcting device,such as the valve II, as already described.

If the object of which the image is to be transmitted bethree-dimensional (having an intensity contrast of unity) the effectiveintensitycontrast of the picture signals may also be reduced to a valuebelow unity before transmission, as already described in connection withthe transmission of film pictures.

In a further method according to the present invention, of transmittingan image of a threedimensional object, the object is photographed, anegative is prepared, a positive is printed and used for transmission.The negative may be developed to a high intensity contrast (that is onein which detail in the darker portions, correspo'nding'to the lighterportions in the positive,

is accentuated), and the positive may be developed to a low intensitycontrast, forexample equal to unity or less. Alternatively the negativemay be developed to a low intensity contrast and the positive may beprepared without change in intensity contrast. a

It has been assumed in the above description that the direct and lowfrequency signal components are arranged to be present wherever a changein intensity contrast of picture signals is effected. This is necessary"for satisfactory results if any appreciable changes in averagebrightness of the object can take place because in the absence of theselow frequency components such changes involve a change in the value, in

terms of picture brightness, of the zero line about which the picturesignals vary, the zero line being a line so placed that the areasenclosed by a the picture signals above and below it are equal.

Changes in the zero line of the signals are equivalent to changes in'thepoint at which the correcting valve is biased and thus the correctionapplied will vary with the average brightness of the object.

The presence of the direct and low frequency components at the pointswhere correction of 'effective intensity contrast talies place can beensured either by proving couplings capable of transmitting thesecomponents (at least up tothe modulating point at the transr'nitterandafter.

' a second output circuit whereby said sound signals are fed to saidsecond output circuit substantially without change in effectiveintensity contrast and means for transmitting the signals from saidoutput'circuits.

2. In the television transmission and reception of motion picturerecords accompanied by sound representation placed upon the film, are

apparatus for increasing the ratio of signal level to noise level whichcomprises means for modifying the intensity contrast of both thesoundand picture portions of the film means for developing electricalimpulses representative of the optical values of both the sound and thepicture representations, means for simultaneously trans- 'mitti'ng therepresentation developed impulses,

means for receiving the impulses transmitted,

means for distorting during reception only the signals representative ofthe optical picture, and

separate means for reproducing both the optical and the sound signalimpulses simultaneously.

3. Combined television and sound transmitting apparatus comprising meansfor deriving picture signals from an object, means for reducing theeffective intensity contrast of said signals comprising an amplifierhaving a curved response characteristic and feeding them to an outputcircuit, means for generating signals corresponding to'sounds,a'coupling between said sound generating means and a second outputcircuit whereby said sound signals are fed to said second output circuitsubstantially without change in effective intensity contrast, and meansfor transmitting the signals from said output circuits.

4. Combined television and sound transmitting I and receiving apparatuscomprising means for deriving picture signals from an object, means forreducing the effective intensity contrast of said signals and feedingthem to an output circuit, means for generating signals corresponding tosounds, a coupling between said sound generating means and a secondoutput circuit whereby said sound signals are fed to said second outputcircuit substantially without change in effective intensity contrast,means for transmitting the signals from said output circuits, means forreceiving said sound and picture signals, means for changing theintensity of contrast of the picture signals only, and means forreproducing the sound signals.

CECIL OSWALD BROWNE. JOHN HARDWICK.

